This invention relates to a method of and apparatus for heating a substrate and more particularly, but not exclusively, to uniformly or substantially uniformly heating at least one surface of the substrate.
It is often the case that, during a manufacturing process, a substrate is required to be heated. For example, when printed circuit boards (PCB""s) are automatically soldered, it is desirable that the temperature of the PCB is raised evenly over its whole surface to avoid undue stress caused by differential temperatures.
In this example, it is necessary to raise the temperature of the PCB quickly and in accordance with a time-temperature profile, such as that shown in the sketch of FIG. 1. Typically, this can result in the required rate of temperature increase to be in the order of 10xc2x0 C. per second.
To obtain the best quality of solder joint, it is also necessary, after soldering, to lower the temperature of the PCB quickly.
Methods commonly available to perform automatic soldering are flow soldering machines, tunnel ovens, batch ovens and vapour phase soldering machines.
Tunnel ovens and batch ovens use a variety of methods to heat PCB""s, including forced air convection and/or radiant heat. Forced air convection is typically implemented by the use of stationery single or multiple circular nozzles directing the forced hot air down onto the surface of the PCB.
The fundamental problem with using circular nozzles in forced air convection is that hot and cold spots are generated across the surface of the PCB. Each nozzle outputs forced hot air directly downwards onto a surface of the PCB, and as the hot air approaches the surface, it tends to spread radially, as seen in FIG. 2. This leads to the area of a PCB 1 directly beneath a circular nozzle 2 being heated quickly and to a relatively high temperature, the heating effect decreasing as the hot air spreads radially outwards, and a cold spot 3 occurring at the point on the PCB 1 that lies on or substantially on the longitudinal axis of the nozzle 2 due to stagnation.
It will be understood that even if a multiplicity of such nozzles is employed that a multiplicity of hot and cold spots will result.
In order to try and improve and evenness of heating of PCB 1 it would be possible to take a single nozzle 2 (FIG. 2) and traverse it along a path as shown in FIG. 3. If the nozzle were moved with sufficient speed then even heating of PCB 1 might result but the necessary traversing mechanism would be complex and expensive.
Similarly it would be possible to traverse the plate holding a multiplicity of nozzles but it would still be difficult to avoid generating hot and cold spots.
The present invention seeks to overcome these problems.
According to a first aspect of the invention, there is provided a method of uniformly or substantially uniformly heating at least one surface of a substrate, the method comprising the steps of:
a) supporting the substrate;
b) generating a heated air flow;
c) directing the heated air flow to heat a strip of a first surface of the substrate; and
d) moving the flow path of the heated air in a direction transverse to the direction in which the heated air flow impinges on the substrate until the temperature of the whole or substantially the whole first surface of the substrate has been uniformly or substantially uniformly raised.
The term xe2x80x98airxe2x80x99 as used hereinafter is intended to also cover nitrogen and any other suitable gas and is to be construed accordingly.
Preferable and/or optional features of the first aspect of the invention are set out in claims 2 to 8, inclusive.
According to a second aspect of the invention, there is provided Apparatus for uniformly or substantially uniformly heating at least one surface of a substrate, the apparatus comprising means for supporting the substrate, means for generating a heated air flow, means for directing the said air flow to heat a strip of a first surface of the substrate, and means for moving the flow path of the heated air until the first surface of the substrate is heated.
Preferable and/or optional features of the second aspect of the invention are set out in claims 10 to 16, inclusive.